1. Field of the Invention
This invention relates to a light beam scanning device which tolerates the influence of an error in inclination of the radiation surface of a rotary mirror or vibrating mirror, and so on, e.g., the inclination assumed by reflection surface of a light beam scanning means with respect to its rotational axis or vibrating axis which causes a positional error in the direction vertical to the scanning direction of the scanning light.
2. Description of the Prior Art
In reading-out and recording operations of image informations, there has been known a method of scanning a read-out surface of an information or a recording surface with a radiation beam. According to this conventional method, there have been used a polygonal mirror or vibrating mirror, etc., as a scanning means to deflect the radiation beam along its scanning direction. However, it is difficult to manufacture the reflection surface of the scanning means in a predetermined, perfect relationship with the rotational axis or the vibrating axis. Even when the above-mentioned reflection surface is maintained at such predetermined, perfect relationship with the above-mentioned rotational axis or vibrating axis, such relationship undergoes variations with lapse of time without the rotational axis of the rotary mirror, or the vibrating axis of the vibrating mirror being able to maintain the definite direction. For instance, in the rotary mirror, there sometimes takes place that the actual rotational axis of the rotary mirror displaces from the regular position of the rotational axis owing to defective dynamic balance of the rotor of a motor which drives the rotational axis. In the above-mentioned case, the scanning light reflected at the reflection surface of the above-mentioned light beam scanning means causes positional displacement on the scanning surface where the writing-in or reading-out operation is to be performed in the direction orthogonal to the scanning direction thereof. For example, in a rotary polygonal mirror, every reflection surface is made in a perfectly parallel relationship with the rotational axis, and in the case of the rotating means being manufactured in high precision, the locus of the beam to be scanned on each of the above-mentioned reflection surfaces is perfectly coincident on the scanning surface. Where each of these reflection surfaces is not in such perfectly parallel relationship with the rotational axis, or the precision in the rotating means is not sufficiently high, the locus of the scanning line on the scanning surface at every reflection surface causes positional discrepancy in the direction orthogonal to the scanning direction of the light beam. Such error of the scanning beam on the scanning surface in the direction orthogonal to the scanning direction thereof has been a great barrier or obstacle in reading-out or recording of high density informations.
Due to such defects in the above-described scanning means, there have been adopted various ways of improving the positional error of the scanning beam on the scanning surface in the direction orthogonal to the scanning direction thereof. Of these various improvements, there are two representative methods, about which explanations will be made in the following.
The first method is to improve the precision in the scanning means per se. For example, every reflection surface in the rotary mirror is brought to a strictly parallel relationship with the rotational axis with high precision, or the precision in the rotary mirror of the rotating means is increased. This method, however, requires extremely precise working to obtain such high precision as required for the reading-out and recording operations of high density information, and to maintain over a long period of time such high precision, hence prohibitive manufacturing cost would incur.
The second method is to use an optical corrective means. For such optical corrective means, there has been a U.S. Pat. No. 3,750,189. According to this method, an incident light beam on the reflection surface of the rotary mirror is focussed on the reflection surface of the above-mentioned rotary mirror by means of a cylindrical lens in the direction where the tilting of the rotary mirror is corrected, in other words, only in the direction orthogonal to the scanning direction of the scanning beam. Accordingly, in an ideal image-forming or focussing state of the incident light beams, a straight line image is formed on the reflection surface of the above-mentioned rotary mirror. Since the optical image which forms an image on the scanning surface from the light beam scanned by the above-mentioned rotary mirror possesses one of the focal plane thereof on the reflection surface of the above-mentioned rotary mirror, and the other focal plane on the above-mentioned scanning surface, the reflection surface of the above-mentioned rotary mirror and the above-mentioned scanning plane is in a co-acting relationship with respect to the above-mentioned image forming optical system. Accordingly, on the reflection surface of the above-mentioned rotary mirror, a component of the scanning light is image-formed with respect to the direction where the inclination of the reflection surface of the rotary mirror is corrected, so that even if there occurs inclination of the reflection surface of the rotary mirror, no influence whatsoever is brought about on the scanning surface in the direction orthogonal to the scanning direction of the scanning beam due to tilting of the reflection surface.
However, even such corrective optical means is not free from various defects. In the first place, since the optical means is so designed that an image may be formed linearly on the reflection surface of the rotary mirror from the scanning light beam, the total light beams are reflected at a very tiny area on a reflection surface. As the result, any surface defect existing on the reflection surface such as very fine dust particles, or scratches, etc., greatly affect the amount of reflective light. In this case, there takes place amplitude modulation in the scanning beam, as a result of which the amount of scanning light becomes irregular on the scanning surface, which is not suitable for practical purposes.
In the second place, since the scanning light is collected at a very tiny area on the reflection surface, the energy density of the beam at this tiny area becomes increased to invite deterioration of the reflection surface due to heat. The resulting reduction in the reflective index on the reflection surface accelerates generation of heat on the reflection surface with the consequent deterioration of the reflection surface.
In the third place, even when the formation of the linear image is intended on the reflection surface as described above, it is practically impossible to constantly obtain such image on the reflection surface, because the reflection surface rotates in its operations. Therefore, the converging state of the light beam on the scanning surface worsens, which is an impermissible defect in the case where the scanning optical system of high resolution is demanded.